1. Field of the invention
The present invention relates to a visual field converting optical system disposed inside flexible or non-flexible endoscopes.
2. Description of the prior art
A conventional visual field converting optical system of this type may utilize fundamentally a reflecting function of a prism or a refracting function of a prism or use an inhomogeneous medium lens.
The optical system utilizing the reflecting function, as shown in, for example, FIG. 1, comprises a plano-concave lens 1, a prism 2 and an objective unit 3 disposed in the distal end portion of an endoscope and an image guide 4 to convert a traveling direction of light into another through the prism 2. Further, as shown in FIG. 2, another optical system arranges prisms 6, 7 in an image transmitting portion 5 of a non flexible endoscope to turn upward a traveling direction of light at an angle of 45.degree. so that the position of an eyepiece portion 8 is convenient for observation. These visual field converting optical systems of the reflection type mentioned above, however, have disadvantages that the number of optical elements to be required is increased and consequently an optical configuration is liable to be complicated, though an angle of deflection can be made considerably wide.
Also, the optical system applying the refracting function, as shown in, for example, FIG. 3, is composed of a prism 9 with a concave lens and an objective unit 3 arranged in the distal end portion of an endoscope and an image guide 4 to deflect a light beam through the prism 9. Such an optical system is often used when the angle of deflection is 20.degree. or less. Further, FIG. 4 shows an example utilizing the refracting function in which an optical axis is inclined in a visual field direction at an angle of approximately 10.degree. with a direction of straight view. This example, which takes account of use of endoscopes in water, is provided with a cover lens 10 arranged perpendicular to the optical axis in front of a prism 9 with a concave lens so that a visual field direction can also be obtained in water in the same manner as in air. However, these visual field converting optical systems of the refraction type create difficulties of distortion and astigmatism caused by the refracting function.
The optical system utilizing both the reflecting and refracting functions, as illustrated in, for example, FIG. 5, has a prism 2 and a prism 9 with a concave lens arranged in the distal end portion of a side-view type endoscope as a back-view type. This visual field converting optical system has problems that the prism 9 projects inevitably into the outside from the side of the endoscope due to its wedge shape and, in case a cover glass is provided to eliminate the projection of the prism, the distal end portion of the endoscope becomes larger in diameter.
The optical system using an inhomogeneous medium lens is described in, for example, Japanese Patent Publication No. Sho 47-28057. This system, as shown in FIGS. 6A and 6B, includes an inhomogeneous medium lens 11 in which refractive index distribution exhibits square distribution (FIG. 6B) and its center axis is curved (FIG. 6A). However, it is very difficult as a matter of fact to make a lens with such refractive index distribution that the center axis is curved. Also, in order to incorporate the lens in the distal end portion of the endoscope, it is necessary to miniaturize the endoscope itself in such a manner that a light beam is turned in a predetermined direction at a short distance. When the extent of the curve of the center axis as well as the variation of the refractive index distribution is made larger for this purpose, the deviation of the position between the centers of the image and the distribution is increased and the resultant image is distorted. Therefore, the above system faces a difficult problem that it is impossible to make large the variation of the refractive index distribution and the curve of the center axis. It is needless to say that the lens with a linear center axis and the refractive index distribution of square distribution in the shape of a wedge has a problem that it is of no practical use since an image forming function with the square distribution is caused asymmetrically with respect to the center axis, in addition to aberration caused by the refracting function on the exit face of the lens, and thereby aberration is largely developed in a direction perpendicular to the optical axis at an image forming position.